This invention relates to wafer-level reliability testing of semiconductor integrated circuit structures at high frequencies. More particularly, such testing is accomplished by generating the high frequencies on-chip to drive the test structures. The frequency and duty cycle of the pulses from the on-chip oscillator are controlled by off-chip DC signals.
As integrated circuits (ICs) continue to be designed with smaller and smaller dimensions, clock speeds continue to rise. CMOS microprocessors operating at more than 200 MHz have been announced. However, typical reliability assessments are based on applying DC and low frequency AC signals to test structures. Since reliability at high frequencies can be different than the results at DC, actual circuit reliability may be severely over or under estimated if high frequency (HF) effects are not considered.
For hot-carrier-induced degradation of MOSFETs, many different AC stress results have been reported. Compared with DC stress where there is little controversy, AC stress results at various laboratories are contradictory. Furthermore, a simple DC stress may not represent the worst-case stress. Alternating injection of holes and the subsequent trapping of electrons can result in additional damage. Mechanical stress has also been shown to enhance AC damage. Other "enhancements" during AC stress have been attributed to instrumentation issues associated with applying HF signals. These contradictory results may also be due, in part, to differences in technology. For electromigration, DC stress is the worst-case stress. A bipolar AC electromigration stress can result in an average failure time that is 1,000 times longer. This amount of "margin" is significant and must be verified for a given technology. For oxide breakdown, an increase in average failure time has been observed under both bipolar and unipolar AC stress. Therefore, it is important to study and characterize these AC reliability effects.
Most AC reliability studies are performed at low frequencies (below 10 MHz). However, today's ICs operate at much faster clock rates. Since most reliability-test systems are DC (or low frequency AC), examining reliability at high frequencies normally requires new and expensive test systems. Studying the reliability over the complete military temperature range (-55.degree. C. to 125.degree. C.) adds additional complexity.